Heat-shrinkable polyester film and package

ABSTRACT

The invention provides a heat-shrinkable polyester film which has sufficient heat shrinkage properties in a main shrinkage direction that is in the longitudinal direction, has low heat shrinkage in the width direction orthogonal to the main shrinkage direction, does not have excessively high shrinkage stress in the main shrinkage direction, and has small attenuation of the shrinkage stress, whereby the followability to a container which is an object to be packaged is high, and slack is less likely to be generated.

TECHNICAL FIELD

The present invention relates to a heat-shrinkable polyester film and a package. More particularly, the present invention relates to a heat-shrinkable polyester film which is suitable for a label application and a banding application to bind a box lunch container or the like and which enables finishing with less slack to an object to be packaged since the attenuation of shrinkage stress of the film at the time of shrinking by heating is small.

BACKGROUND ART

Recently, in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing, and accumulation package, there have been widely used drawn films (so-called heat-shrinkable films) composed of a polyvinyl chloride resin, a polystyrene resin, a polyester resin or the like. Of these heat-shrinkable films, a polyvinyl chloride film has problems that heat resistance is low, and it generates hydrogen chloride gas in incineration and causes dioxin. A polystyrene film has problems that it is inferior in solvent resistance, as well as an ink with a special composition needs to be used in printing, it requires high temperature incineration and generates a lot of black smoke accompanied by an abnormal odor. Therefore, as a shrink label, there has been widely used a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance, and the use amount tends to increase being accompanied by an increase in distribution volume of PET containers.

As an ordinary heat-shrinkable polyester film, one which is allowed to shrink greatly in the width direction has been widely utilized. When the film is used as a label film for a bottle or a banding film for binding a box lunch container or the like, the film should be made into an annular form, mounted to the bottle or the box lunch container or the like, and then allowed to heat-shrink in the circumferential direction. Therefore, when a heat-shrinkable film that heat-shrinks in the width direction is mounted as a banding film, after forming an annular-shaped member such that the width direction of the film is in the circumferential direction, the annular-shaped member should be cut into segments having a predetermined length, and each segment should be mounted to the bottle or the box lunch container, for example, by placing it by hand over the bottle or the box lunch container. Therefore, it is difficult to mount a label or a banding film made of the heat-shrinkable film that heat-shrinks in the width direction to a bottle or a box lunch container at high speed. For that reason, recently, there is a need for a longitudinally heat-shrinkable film which can be wound around a bottle or a box lunch container directly from a film roll to mount the bottle or the box lunch container. With such a heat-shrinkable film, a center sealing step in which an annular-shaped member is formed and sealed or processing such as cutting, placing by hand, or the like can be eliminated, and thus mounting at high speed is also possible.

As a demand for a shrinkable film, it is desired to follow the shape of an object to be packaged at the time of shrinking and to have a tight feeling after shrinking. In the case of a label of a beverage bottle, if the label does not follow the shape of the bottle and has no tightness, there is a problem that when a consumer holds the body of the bottle and opens the cap of the bottle, the cap cannot be opened easily since the label rotates. Furthermore, in the case of a banding application of a box lunch container, the shrinkable film is required to be tightly finished to prevent spilling of the contents in the box lunch container and to prevent foreign matter contamination.

As a method for giving tightness after finishing shrinkage to a shrinkable film, it is conceivable to increase shrinkage stress. However, if the shrinkage stress is too high, there is a problem that when a thin soft beverage bottle or box lunch container is packaged with the film, the bottle or container may deform. In addition, there is a problem that the bonded portion of a cylindrical label or a banding film may be separated due to high shrinkage stress.

For example, in Patent Document 1, it is described that a heat-shrinkable film is subjected to intermediate heat treatment after drawing in the longitudinal direction, and then drawn in the width direction, whereby the maximum shrinkage stress in the width shrinkage direction of the film when measured in hot air at 90° C. is large, and the shrinkage stress does not attenuate much after 30 seconds, and it is described that such a heat-shrinkable film is satisfactory in followability to a container in a label application, the slack of the label after finishing is less likely to be generated, and a satisfactory appearance can be attained.

However, in the method described in Patent Document 1, a large-scale facility for biaxially drawing is necessary, which causes a problem of increased cost. Furthermore, it is difficult to mount a film that shrinks in the width direction as described above to a bottle or a box lunch container at high speed.

PRIOR ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Patent Publication No. 5240387 (B1)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a heat-shrinkable polyester film which has sufficient heat shrinkage properties in a main shrinkage direction that is in the longitudinal direction, has low heat shrinkage in the width direction orthogonal to the main shrinkage direction, does not have excessively high shrinkage stress in the main shrinkage direction, and has small attenuation of the shrinkage stress, whereby the followability to a container which is an object to be packaged is high, and slack is less likely to be generated.

Means for Solving the Problem

That is, the present invention has the following constitution.

1. A heat-shrinkable polyester film which has a main shrinkage direction in a longitudinal direction of the film and satisfies the following requirements (1) to (4):

(1) the film has a hot-water shrinkage in the main shrinkage direction of the film of 40% or more and 80% or less when treated for 10 seconds in hot water of 98° C.;

(2) the film has a hot-water shrinkage in a direction orthogonal to the main shrinkage direction of the film of −5% or more and 15% or less when treated for 10 seconds in hot water of 98° C.;

(3) with regard to a shrinkage stress in the main shrinkage direction of the film measured under hot air of 90° C., the film has a shrinkage stress ratio of 0.6 or more and 1.0 or less, the shrinkage stress ratio being represented by the following equation:

shrinkage stress ratio=(shrinkage stress after 30 seconds)÷(maximum shrinkage stress); and

(4) the film has a refractive index in the main shrinkage direction of the film of 1.600 or more.

2. The heat-shrinkable polyester film according to the above 1, wherein a maximum shrinkage stress in the main shrinkage direction of the film measured under hot air of 90° C. is 15 MPa or less.

3. The heat-shrinkable polyester film according to the above 1 or 2, wherein the film comprises ethylene terephthalate as a main constituent component and 2 mol % or more of at least one monomer component that can form an amorphous component in a whole polyester resin component.

4. The heat-shrinkable polyester film according to any of the above 1 to 3, wherein neopentyl glycol is used as at least one monomer component that can form an amorphous component.

5. A package obtained by covering at least a part of an outer periphery of an object to be packaged with the heat-shrinkable polyester film according to any of the above 1 to 4 and then shrinking the film on the covered object by heat.

Effects of the Invention

The present inventors made intensive studies, and as a result, have found that in a single layer film made of a single resin or a laminated film in which different types of resins are laminated, using at least one layer made of a polyester resin having a specific composition allows the shrinkage stress in the main shrinkage direction to be not too high and allows the attenuation of the shrinkage stress to be small, and thus the film has high followability to a container which is an object to be packaged at the time of shrinking and is less likely to slacken. The present invention has been completed based on this finding.

That is, according to the present invention, it is possible to provide a heat-shrinkable polyester film which has sufficient heat shrinkage properties in a main shrinkage direction that is in the longitudinal direction, has low heat shrinkage in the width direction orthogonal to the main shrinkage direction, does not have excessively high shrinkage stress in the main shrinkage direction, and has small attenuation of the shrinkage stress, whereby the followability to a container which is an object to be packaged is high at the time of shrinking, and slack is less likely to be generated. The heat-shrinkable polyester film of the present invention can be suitably used as a film label for a bottle or a banding film to bind a container such as a box lunch and can be very efficiently mounted to the bottle or the container within a short time since the main shrinkage direction is in the longitudinal direction. In addition, when the heat-shrinkable polyester film of the present invention is subjected to heat shrinkage after mounting, shrinkage shortage, lengthwise sink-marks and container deformation are extremely reduced, and the followability to the container is sufficient and slack is less likely to be generated due to small attenuation of the shrinkage stress, whereby good finishing can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a box lunch container evaluating wrinkles of film after shrinking.

FIG. 2 shows a box lunch container evaluating sink marks of film after shrinking.

EXPLANATION OF LETTERS OR NUMERALS

-   -   1: box lunch container     -   2: film     -   3: wrinkle     -   4: box lunch container     -   5: film

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the structure of the heat-shrinkable polyester film according to the present invention will be described.

The heat-shrinkable polyester film according to the present invention has at least one layer containing 50 mol % or more of an ethylene terephthalate unit in 100 mol % of the constituent units of polyester. The details will be described later, but as a result of studies made by the present inventors, it has been found that as for a film having at least one layer containing 50 mol % or more of an ethylene terephthalate unit in 100 mol % of the constituent units of polyester, if the draw ratio is increased more than 3 times, the crystallization is promoted, and consequently the attenuation rate of the shrinkage stress is small and the shrinkage stress after 30 seconds from the initiation of shrinking becomes high.

[Case of Single Layer Film]

In the case of a single layer film, for the reason mentioned above, the polyester used in the heat-shrinkable polyester film is a polyester containing an ethylene terephthalate unit as a main constituent. The content of the ethylene terephthalate unit is 50 mol % or more in 100 mol % of the constituent units of the polyester. In order to promote crystallization at the time of drawing in the longitudinal direction as described later, the content of the ethylene terephthalate unit is preferably 55 mol % or more, and more preferably 60 mol % or more in 100 mol % of the constituent units of the polyester. However, if the content of the ethylene terephthalate unit is too high, it is difficult to obtain necessary high shrinkage because shrinkage property is inhibited by crystallization, and therefore the upper limit of the content of the ethylene terephthalate unit is preferably 70% or less.

Other dicarboxylic acid components constituting the polyester of the present invention can include aromatic dicarboxylic acids such as isophthalic acid, naphthalenedicarboxylic acid, and ortho-phthalic acid, and; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acid; or the like.

When an aliphatic dicarboxylic acid (for example, adipic acid, sebacic acid, decanedicarboxylic acid or the like) is contained in the polyester, the content is preferably less than 3 mol % (in 100 mol % of the dicarboxylic acid component).

Further, it is preferable not to contain polybasic carboxylic acids of tribasic or more (for example, trimellitic acid, pyromellitic acid and anhydride thereof etc.) in the polyester. A heat-shrinkable polyester film obtained by using a polyester containing these polybasic carboxylic acids is hard to achieve a necessary high shrinkage.

Diol components constituting the polyester include aliphatic diols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and hexanediol; alicyclic diols such as 1,4-cyclohexanedimethanol; and aromatic diols such as bisphenol A.

Further, the polyester has 2% or more of the sum of at least one monomer component that can form an amorphous component in 100 mol % of a polyhydric alcohol component or in 100 mol % of a polybasic carboxylic acid component in the whole polyester resin, preferably 3% or more, more preferably 4% or more, and particularly preferably 5% or more. If the amount of monomer component that can form an amorphous component increases, crystallization at the time of drawing in the longitudinal direction does not sufficiently proceed, and therefore the upper limit is preferably 20 mol %.

Examples of the monomer that can form an amorphous component may include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol, or isophthalic acid is preferably used. In addition, ε-caprolactone is also preferably used.

To a resin for forming the heat-shrinkable polyester film of the present invention, according to needs, there can be added various additives, such as waxes, an antioxidant, an antistatic agent, a crystal-nucleation agent, a viscosity reducing agent, a heat stabilizer, a pigment for coloring, a color protection agent, and an ultraviolet absorber.

By adding fine particles as lubricant to a resin for forming the heat-shrinkable polyester film of the present invention, it is preferable to make workability (slipperiness) of the film better. The fine particles can be arbitrarily selected, for example, as inorganic fine particles, silica, alumina, titanium dioxide, calcium carbonate, kaolin, barium sulfate and the like can be listed. As organic fine particles, for example, an acrylic resin particle, a melamine resin particle, a silicone resin particle, a crosslinked polystyrene particle and the like can be listed. The average particle diameter of the fine particles is in a range of 0.05 to 3.0 μm (when measured by coulter counter), and it can be suitably selected according to need.

As a method for compounding the above-described particles in a resin for forming the heat-shrinkable polyester film, for example, they can be added in an arbitrary step in production of the polyester resin, but they are preferably added in a step of esterification, or in a step after completion of ester exchange reaction and before start of polycondensation reaction as slurry dispersed in ethylene glycol etc., followed by carrying out polycondensation reaction. Further, it is also preferably carried out by a method in which slurry of particles dispersed in ethylene glycol, water or the like and raw materials of polyester resin are mixed using a kneading extruder with a vent, or a method in which dried particles and raw materials of polyester resin are mixed using a kneading extruder.

It is also possible to conduct corona treatment, coating treatment, frame treatment etc. on the heat-shrinkable polyester film of the present invention in order to enhance adhesiveness of film surface.

[Case of Laminated Film]

In the case of a laminated film in which resin layers having different resin compositions are laminated, it is necessary to use at least one polyester layer containing 50 mol % or more of an ethylene terephthalate unit in 100 mol % of the constituent units of the polyester in the film laminate structure. For the same reason as in the case of the single layer film, that is, for the reason that by providing at least one layer containing 50 mol % or more of the ethylene terephthalate unit in the film structure of the laminated film, the laminated film has characteristics that if the draw ratio is increased more than 3 times, the crystallization is promoted, and consequently the attenuation rate of the shrinkage stress is small and the shrinkage stress after 30 seconds from the initiation of shrinking becomes high.

In the present invention, when the film is made into a three-layer structure, it is preferable that an outermost layer (skin layer) be a layer containing 50 mol % or more of an ethylene terephthalate unit. The reason for this is to promote the crystallization of the outermost layer by drawing and reduce the attenuation rate of the shrinkage stress.

The composition of a resin for forming a core layer is not particularly limited. However, from the viewpoint of mechanical strength and the like, a resin containing an ethylene terephthalate unit as a main constituent component is preferable, and the content of the ethylene terephthalate unit is preferably 85 mol % or less in 100 mol % of the constituent units of the polyester. If the content of the ethylene terephthalate unit is too large, the crystallization is excessively promoted, and hence a high shrinkage cannot be obtained.

Diol components constituting the polyester in the core layer include aliphatic diols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and hexanediol; alicyclic diols such as 1,4-cyclohexanedimethanol; and aromatic diols such as bisphenol A.

Further, the polyester has 2% or more of the sum of at least one monomer component that can form an amorphous component in 100 mol % of a polyhydric alcohol component or in 100 mol % of a polybasic carboxylic acid component in the whole polyester resin, preferably 3% or more, more preferably 4% or more, and particularly preferably 5% or more.

Here, the interpretation of the term “can form an amorphous component” is described in detail.

In the present invention, the “amorphous polymer” specifically refers to the case where no endothermic peak due to fusion is shown in measurement with a differential scanning calorimeter (DSC). Since the crystallization of the amorphous polymer does not substantially proceed, the amorphous polymer cannot be in a crystalline state or has an extremely low degree of crystallinity even when crystallized.

Furthermore, in the present invention, the “crystalline polymer” refers to a polymer other than the above-mentioned “amorphous polymer”, that is, the case where an endothermic peak due to fusion is shown in measurement with a differential scanning calorimeter (DSC). The crystalline polymer means a polymer that can be crystallized when heated, has a crystallizable property, or has been already crystallized.

In general, as for a polymer being in a state where a plurality of monomer units are bonded, when the polymer has various conditions such as low stereoregularity of a polymer, poor symmetry of a polymer, a large side chain of a polymer, a large number of branches of a polymer, and low intermolecular cohesion between polymers, the polymer becomes amorphous. However, depending on the existence state, the crystallization sufficiently proceeds, and the polymer may become crystalline. For example, even for a polymer having a large side chain, when the polymer is composed of a single monomer unit, the crystallization of the polymer may sufficiently proceed, and the polymer may become crystalline. For this reason, even if the polymer is composed of the same monomer unit, the polymer can become crystalline or can become amorphous, and therefore in the present invention, the expression “unit derived from a monomer that can form an amorphous component” is used.

The monomer unit in the present invention means a repeating unit constituting a polymer induced from one polyhydric alcohol molecule and one polybasic carboxylic acid molecule, and in the case of ε-caprolactone, means a constituent unit obtained by opening the lactone ring.

When a monomer unit composed of terephthalic acid and ethylene glycol is a main monomer unit constituting a polymer, examples of the above unit derived from a monomer that can form an amorphous component include a monomer unit composed of isophthalic acid and ethylene glycol, a monomer unit composed of terephthalic acid and neopentyl glycol, a monomer unit composed of terephthalic acid and 1,4-cyclohexanedimethanol, and a monomer unit composed of isophthalic acid and butanediol, or the like.

Examples of the monomer that can form an amorphous component of a resin for forming a core layer may include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol, or isophthalic acid is preferably used.

It is desirable that the value obtained by dividing the sum of the thicknesses of the skin layers by the thickness of the core layer be 0.1 to 0.5. If the value obtained by dividing the sum of the thicknesses of the skin layers by the thickness of the core layer is less than 0.1, the amount of the layer containing 50 mol % or more of a polyethylene terephthalate unit in the film structure of the laminated film is small, and this is unfavorable since the effect of reducing the attenuation rate of the shrinkage stress cannot be sufficiently obtained. On the other hand, if the value obtained by dividing the sum of the thicknesses of the skin layers by the thickness of the core layer exceeds 0.5, the amount of the core layer which mainly performs a heat shrinkage behavior is relatively too small, and hence this is unfavorable since the necessary heat shrinkage cannot be achieved.

In the heat-shrinkable polyester film of the present invention, each of the core layer and the skin layers preferably has a thickness of 1 μm or more. It is not preferred that the thickness of any of the core layer and the skin layers be less than 1 μm since the necessary shrinkage properties cannot be obtained.

To a resin for forming any of the skin layers and the core layer, according to needs, there can be added various additives such as waxes, an antioxidant, an antistatic agent, a crystal-nucleation agent, a viscosity reducing agent, a heat stabilizer, a pigment for coloring, a color protection agent, and an ultraviolet absorber.

The laminated film can be produced by a known method used for producing a laminated film, and a feed block method, a multi-manifold method, and the like can be given. For example, in the case of the co-extrusion method, various resin mixtures for forming layers are separately melted using an extruder, merged in a T-die mold equipped with a multi-manifold and extruded, and drawn by a drawing apparatus, whereby a laminated film can be obtained.

Although the form of the laminated film is not particularly limited, for example, a two-type two-layer structure of A/B, a two-type three-layer structure of B/A/B, and a three-type three-layer structure of C/A/B can be exemplified.

[Properties of Heat-Shrinkable Polyester Film of the Present Invention]

When the heat-shrinkable polyester film of the present invention is treated for 10 seconds in no load state in hot water of 98° C., a heat shrinkage in the longitudinal direction which is the main shrinkage direction of the film calculated from the lengths before and after shrinkage according to the following Equation 1 (namely, hot-water heat shrinkage at 98° C.) is 40% or more and 80% or less.

Heat shrinkage={(length before shrinkage−length after shrinkage)/length before shrinkage}×100(%)  Equation 1

If the hot-water heat shrinkage in the longitudinal direction at 98° C. is less than 40%, the shrinkage amount is small in the case of using the film as a banding film, so that wrinkles and slack are generated on a label after heat shrinkage, and therefore this is not-unfavorable. On the other hand, if the hot-water heat shrinkage in the longitudinal direction at 98° C. is more than 80%, when a thin soft beverage bottle or box lunch container is packaged with the film, a problem of deformation of the bottle or container arises. In addition, there is a problem that the bonded portion of a cylindrical label or a banding film is separated due to high shrinkage stress. The hot-water heat shrinkage in the longitudinal direction is more preferably 75% or less, and further preferably 70% or less. Incidentally, the lower limit of the hot-water heat shrinkage in the longitudinal direction at 90° C. is more preferably 45% or more, and further preferably 50% or more.

Furthermore, when the heat-shrinkable polyester film of the present invention is treated for 10 seconds in no load state in hot water of 98° C., a hot-water heat shrinkage in the width direction which is a direction orthogonal to the main shrinkage direction of the film calculated from the lengths before and after shrinkage according to the above Equation 1 is −5% or more and 15% or less. If the hot-water heat shrinkage in the width direction at 98° C. exceeds 15%, when the film is used as a banding film, the length of the film in the direction orthogonal to the shrinkage direction decreases at the time of heat shrinking (occurrence of a sink mark), thereby causing such problems that the contents of a box lunch spill out and foreign matters are mixed into the box lunch due to a decrease in banding force, and therefore this is not unfavorable. On the other hand, if the hot-water heat shrinkage in the width direction at 98° C. is less than −5%, the length of the label in the direction orthogonal to the main shrinkage direction increases at the time of heat shrinking, the label is slackened and likely to wrinkle, and therefore this is not unfavorable. Incidentally, the hot-water heat shrinkage in the width direction at 98° C. is preferably −4% or more and 9% or less, more preferably −3% or more and 8% or less, and further more preferably from −2% or more and 7% or less.

In the heat-shrinkable polyester film of the present invention, with regard to the shrinkage stress in the main shrinkage direction of the film measured under hot air of 90° C., the shrinkage stress ratio represented by the following equation is 0.6 or more and 1.0 or less.

Shrinkage stress ratio=(shrinkage stress after 30 seconds)÷(maximum shrinkage stress)

That is, the heat-shrinkable polyester film of the present invention features specific heat shrinkage properties such that the shrinkage stress almost comparable to the maximum heat shrinkage stress is developed even after 30 seconds from the initiation of shrinking by heat. If the shrinkage stress after 30 seconds/maximum shrinkage stress (hereinafter, stress ratio) of the film is less than 0.6, in the case of a label of a beverage bottle, the label does not follow the shape and has no tightness, thereby causing a problem that when a consumer holds the body of the bottle and opens the cap of the bottle, the cap cannot be opened easily since the label rotates, and therefore this is not unfavorable. Furthermore, in the case of a banding application of a box lunch container, the shrinkable film is not tightly finished, thereby causing such problems that the contents of the box lunch spill out and foreign matters are mixed into the box lunch. The above-mentioned stress ratio is more preferably 0.75 or more, and further preferably 0.8 or more. Although a higher stress ratio is preferred because the followability is more improved, it is improbable that the shrinkage stress after 30 seconds exceeds the maximum shrinkage stress, and therefore the maximum value of the above-mentioned stress ratio is 1.

The heat-shrinkable polyester film of the present invention has a refractive index of 1.600 or more in the longitudinal direction which is the main shrinkage direction of the film. It is not preferable that the refractive index in the longitudinal direction be less than 1.600 because the film has no rigidity (stiffness feeling) and is likely to wrinkle when formed into a label. The lower limit of the refractive index in the longitudinal direction is preferably 1.625 or more, and more preferably 1.650 or more. On the other hand, it is not preferable that the refractive index in the longitudinal direction exceed 1.700 because the solvent adhesiveness deteriorates when forming a label.

In the heat-shrinkable polyester film of the present invention, when a heat shrinkage stress in the longitudinal direction which is the main shrinkage direction of the film is measured under the conditions of a test piece width of 20 mm and a distance between chucks of 100 mm in hot air of 90° C. at the blowing speed of 5 m/sec, the maximum heat shrinkage stress is preferably 15 MPa or less. If the maximum heat shrinkage stress is 15 MPa or less, the shrinkage stress is not too high, and when a thin soft beverage bottle or box lunch container is packaged with the film, a problem of deformation of the bottle or container does not occur. This is also preferred because a problem such that the bonded portion of a cylindrical label or a banding film is separated due to high shrinkage stress does not arise. The maximum heat shrinkage stress is more preferably 14 MPa or less, and further preferably 12 MPa or less. On the other hand, if the above-mentioned shrinkage stress is too small, when a beverage bottle or a box lunch container is packaged, the tightness after finishing shrinkage is insufficient, thereby causing a problem that when a consumer holds the body of the bottle and opens the cap of the bottle, the cap cannot be opened easily because the label rotates. Furthermore, in the case of a banding application of a box lunch container, problems such that the contents of the box lunch spill out and foreign matters are mixed into the box lunch arise. Therefore, the maximum heat shrinkage stress measured by the above method is preferably 5 MPa or more, and more preferably 6 MPa or more.

The thickness of the heat-shrinkable polyester film of the present invention is not particularly limited, but as a heat-shrinkable film for a label application and a banding application, the thickness is preferably 5 to 100 μm, and more preferably 10 to 95 μm.

The heat-shrinkable polyester film of the present invention is not particularly limited in its production method, but the film can be obtained, for example, by melt-extruding the above-mentioned polyester raw material with an extruder to form an undrawn film and by drawing the undrawn film with a method as shown below.

When a raw material resin is melt-extruded, it is preferable to dry the polyester raw material using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After the polyester raw material is dried in such a manner, it is melted at a temperature of 200 to 300° C. and extruded into a film form utilizing an extruder. In such an extrusion, an arbitrary conventional method such as a T-die method and a tubular method can be adopted.

Then, the sheet-like molten resin after extrusion is quenched so that an undrawn film can be obtained. As a method for quenching the molten resin, a method in which a molten resin is cast on a rotary drum from a spinneret and solidified by quenching to obtain a substantially unoriented resin sheet can be suitably adopted.

Further, the obtained undrawn film is drawn in the longitudinal direction under a predetermined condition as described below so that the heat-shrinkable polyester film of the present invention can be obtained. Hereinafter, a preferable drawing to obtain the heat-shrinkable polyester film of the present invention will be described in detail while taking into consideration the difference from a method for drawing a conventional heat-shrinkable polyester film.

[Preferable Drawing Method of Heat-Shrinkable Polyester Film]

An ordinary heat-shrinkable polyester film is produced by drawing an undrawn film in a direction to be shrunk. Conventionally, there has been a high demand for a heat-shrinkable polyester film shrinkable in the longitudinal direction which has followability to the shape of an object to be packaged at the time of shrinking and a tight feeling after shrinking. However, it is impossible to obtain a polyester film having a low attenuation rate of shrinkage stress and having a high shrinkage stress after 30 seconds from the initiation of shrinking merely by drawing the undrawn film in the longitudinal direction.

Here, a preferable drawing method of the heat-shrinkable film of the present invention is described.

As a result of studies, the inventors have found that as for a film having at least one layer containing 50 mol % or more of an ethylene terephthalate unit in 100 mol % of the constituent units of polyester, if the draw ratio is increased more than 3 times, the crystallization is promoted, and consequently the attenuation rate of the shrinkage stress is small and the shrinkage stress after 30 seconds from the initiation of shrinking becomes high.

In a conventional heat-shrinkable polyester film containing an amorphous component in a large amount, if the film is subjected to an only uniaxial drawing, the attenuation rate of the shrinkage stress is large, and the shrinkage stress after 30 seconds from the initiation of shrinking reduces. On the other hand, in the present invention, the heat-shrinkable polyester film has at least one layer containing 50 mol % or more of an ethylene terephthalate unit, and the attenuation rate of the shrinkage stress is reduced by drawing at a high ratio of 3 times or more. It is considered that such an attenuation of the shrinkage stress is associated with crystallization due to drawing. In the polyester film of the present invention which has at least one layer containing 50 mol % or more of the ethylene terephthalate unit and is drawn at a high ratio of 3 times or more, molecules are easily crystallized. It is considered that this crystal of the molecules is low in mobility at the time of heating compared with amorphous molecules and suppresses sudden relaxation of molecular orientation when the film shrinks by heat, and therefore slow relaxation of molecular orientation occurs. In other words, it is considered that since the relaxation of orientation occurs for a long time, the attenuation rate of the shrinkage stress lower, and the shrinkage stress after 30 seconds becomes high. When the film contains a large amount of amorphous components, or when the draw ratio in the longitudinal direction is less than 3 times, crystallization does not relatively proceed, and hence it is assumed that the attenuation rate of the shrinkage stress in the longitudinal direction becomes high and the shrinkage stress after 30 seconds reduces.

Based on the results of studies as described above, the draw ratio in the longitudinal direction is preferably 3 times or more and 7 times or less. It is not preferable that the draw ratio in the longitudinal direction be less than 3 times because the crystallization of the film is insufficient, the shrinkage stress does not persist, and as a result, the film does not adequately follow the shape of an object to be packaged, leading to the occurrence of defects such as wrinkles when the film is shrunk as a label or a banding film. This is also not preferable because the irregularity of thickness in the lengthwise direction of the film increases. The upper limit of the lengthwise draw ratio is not particularly limited, but the draw ratio of more than 7 times is not preferable because it becomes difficult to draw the film in the longitudinal direction (so-called breakage tends to occur). The draw ratio is more preferably 3.2 times or more and 6.5 times or less, and further preferably 3.5 times or more and 6 times or less.

The package of the present invention is a package in which a banding film (and a label) obtained by using the heat-shrinkable polyester film of the present invention is covered at least on a part of the outer periphery of an object to be packaged and then to shrink by heat. The object to be packaged can be exemplified by PET bottles for beverage, various kinds of bottles, cans, plastic containers for confectionary, a box lunch and the like, paper-made boxes, and the like. In general, in the case where a label obtained by using a heat-shrinkable polyester film is covered on the packaging object and heat-shrunk, the banding film (and a label) is heat-shrunk by about 5 to 70% and closely attached on the package. Additionally, a banding film (and a label) covered on a packaging object may be printed or may not be printed.

A method for producing a banding film (and a label) is as follows; a rectangular film is rounded in the longitudinal direction to stack the end parts and bonded into a label-form, or a film wound as a roll is rounded to stack the end parts and bonded into a tube-form, which is cut into a label. As a method for bonding the films together, a known method such as fusion sealing, solvent bonding, bonding with hot-melt adhesive, and bonding with an energy ray-curable adhesive can be used.

EXAMPLES

Hereinafter, the present invention is described in more detail by Examples, but the present invention is by no means limited to aspects of the Examples, and it can be suitably modified in the range not departing from the scope of the present invention. The composition of the raw materials used in Examples and Comparative Examples is shown in Table 1. The ratio of the mixed raw material used in each layer is shown in Table 2. The production condition and the result of the evaluation for the films of Examples and Comparative Examples is shown in Table 3.

TABLE 1 composition of polyester raw material (mol %) polyester acid polyhydric alcohol addition raw component component amount of material TPA EG BD NPG DEG lubricant 1 100 99 — — 1 2 100 99 — — 1 7200 3 100 68 — 30 2 4 100 — 100 — —

TABLE 2 ratio of polyester raw material (wt %) mixed raw material 1 2 3 4 A 45 5 50 0 B 70 5 25 0 C 25 5 60 10 D 5 5 66 24

Evaluation methods for films are as follows.

[Tg (Glass Transition Point)]

Using a differential scanning calorimeter (manufactured by Seiko Instruments Inc., DSC220), 5 mg of an undrawn film was put in a sample pan, the lid of the pan was closed, and the temperature was raised at a temperature rising speed of 10° C./minute from −40° C. to 120° C. in a nitrogen gas atmosphere to conduct measurement. Tg (° C.) was obtained based on JIS-K 7121-1987.

[Intrinsic Viscosity (IV)]

0.2 g of a polyester was dissolved in 50 ml of a solvent mixture of phenol/1,1,2,2-tetrachloroethane (60/40 (weight ratio)), and the intrinsic viscosity was measured at 30° C. using an Ostwald viscometer. The unit is dl/g.

[Heat Shrinkage (Hot-Water Heat Shrinkage)]

A film was cut into a square of 10 cm×10 cm, dipped in hot water of a predetermined temperature ±0.5° C. in no load state for 10 seconds to be heat-shrunk, then dipped in water of 25° C.±0.5° C. for 10 seconds, and taken from water. Then, the dimensions of the film in the lengthwise and transverse directions were measured, and heat shrinkage each was obtained according to the following Equation 1. The direction with the large heat shrinkage was defined as a main shrinkage direction.

Heat shrinkage={(length before shrinkage−length after shrinkage)/length before shrinkage}×100(%)  Equation 1

[Shrinkage Stress]

A rectangular film sample of 150 mm in length in the main shrinkage direction and 20 mm in width was cut out of a heat-shrinkable film and measured for the shrinkage stress using a strength and elongation measuring machine with a heating furnace (TENSILON (registered trade mark of Orientec Co., Ltd) universal testing instrument PTM-250) manufactured by Toyo Baldwin Co. (current company name: Orientec Co., Ltd). The heating furnace of the strength and elongation measuring machine was previously heated to 90° C., and the distance between chucks for holding the film sample was set to 100 mm. When the sample was fitted to the chucks of the strength and elongation measuring machine, the air blast blown into the heating furnace was once stopped, the door of the heating furnace was opened, 25 mm of both edges of the sample of 150 mm in the longitudinal direction were clipped with the respective chucks, the distance between the chucks was set to 100 mm, and the sample was fixed without looseness such that the longitudinal direction of the sample was conformed to the direction between the chucks and the sample became horizontal. After fixing the sample to the chucks, the door of the heating furnace was quickly closed, and the air blast was restarted. The point of time when the door of the heating furnace was closed and the air blast was restarted was taken as a measurement start point of shrinkage stress, and the shrinkage stress (MPa) after 30 seconds was obtained. The maximum value of the shrinkage stress measurement values from the measurement start point of shrinkage stress to 30 seconds after the start of measurement was taken as a maximum value of shrinkage stress (maximum shrinkage stress (MPa)). Note that when the shrinkage stress was measured, the distance between the chucks was fixed to 100 mm and the transition of shrinkage stress from the start of measurement to 30 seconds after the start of measurement was measured. The ratio of a shrinkage stress after 30 seconds from the measurement start point relative to the maximum shrinkage stress was defined as a shrinkage stress ratio (represented by the following equation).

Shrinkage stress ratio=(shrinkage stress after 30 seconds)÷(maximum shrinkage stress)

[Refractive Index]

Using “Abbe refractometer 4T type” manufactured by Atago Co. Ltd., the measurement was carried out after each sample film was left in an atmosphere of 23° C. and 65% RH for 2 hours or longer.

[Shrinkage Finishing Property (Wrap Round)]

A film of 50 mm in width was wrapped around a plastic container (side: 150×150 mm, height: 100 mm) of a box lunch so as to bundle the body part and the lid part of the container such that the circumferential direction of the container corresponds to the shrinkage direction of the film. After fusion-sealing at 220° C., the film was heat-shrunk to the plastic container of a box lunch in a shrink tunnel of a preset temperature of 90° C. For shrinkage finishing property, evaluation was made on four points of wrinkles, sink marks, shrinkage shortage and slack. As to the evaluation of wrinkles, as shown in FIG. 1, the judgement was made based on the number of wrinkles of 5 cm or more in length formed in the side direction of the box lunch container. The criteria were as follows.

Good: 0 to 4 wrinkles Fair: 5 to 14 wrinkles Poor: 15 or more wrinkles

With regard to sink marks, FIG. 2 shows a top view of a box lunch container with a banding film after shrinking, and “L” indicates a length from the edge of one side of the film to the edge of the other side of the film. When the length “L” was measured at 5 mm pitch in the circumferential direction of the box lunch container, the difference between the maximum value Lmax and the minimum value Lmin of the lengths was taken as “R”. One having a large “R” was judged as having a large sink mark. The criteria were as follows.

Good: 0 mm≤R<10 mm Fair: 10 mm≤R<15 mm Poor: 15 mm≤R

As to shrinkage shortage, the judgement was made based on whether shrinkage shortage occurred after shrinkage finishing. The criteria were as follows.

Good: No shrinkage shortage occurred. Poor: Shrinkage shortage occurred.

As to slack, the case where the banding film was not completely brought into intimate contact with the box lunch container, had no tightness when touched with hands, and was floating was judged as having slack. The criteria were as follows.

Good: The film was tightly fitted and not floating. Poor: The film was loosely finished and floating.

Preparation of Polyester Raw Material Synthetic Example 1

100 mol % of dimethyl terephthalate (DMT) as a dicarboxylic acid component and 100 mol % of ethylene glycol (EG) as a polyhydric alcohol component were placed in a stainless steel autoclave equipped with a stirrer, a thermometer and a partially circulating cooler such that the amount of ethylene glycol was 2.2 times the amount of dimethyl terephthalate in terms of the molar ratio, 0.05 mol % (based on the acid component) of zinc acetate was added as an ester exchange catalyst, 0.225 mol % (based on the acid component) of antimony trioxide was added as a polycondensation catalyst, and an ester exchange reaction was carried out while distilling away generated methanol to outside the system. Thereafter, a polycondensation reaction was carried out at 280° C. under a reduced pressure of 26.7 Pa to obtain polyester 1 having an intrinsic viscosity of 0.75 dl/g. The composition is shown in Table 1.

Synthetic Examples 2 to 7

Polyesters 2 to 4 shown in Table 1 were prepared in the same manner as in Synthetic Example 1. In the production of polyester 2, SiO2 (Silysia 266, manufactured by FUJI SILYSIA CHEMICAL LTD.; average particle diameter: 1.5 μm) was added as a lubricant at a proportion of 7,200 ppm relative to the polyester. In the Table, NPG is neopentyl glycol, BD is 1,4-butanediol, and DEG is diethylene glycol, which is a side product. The intrinsic viscosities of polyesters 2, 3 and 4 were 2: 0.75 dl/g, 3: 1.20 dl/g and 4: 1.20 dl/g, respectively. Each polyester was appropriately formed into a chip.

Example 1

Polyester 1, polyester 2 and polyester 3 as described above were mixed in the mass ratio of 45:5:50 and the mixed resin was introduced into an extruder. The mixed resin was molten at 280° C. and extruded from a T-die and then quenched by winding it around a rotating metal roll set at a surface temperature of 30° C. to obtain an undrawn film with a thickness of 42 μm. Tg of the undrawn film was 75° C. The obtained undrawn film was introduced to a lengthwise drawing machine in which a plurality of rolls were continuously disposed, heated till the film temperature reached 80° C. on a preheating roll, and then lengthwise drawn by a roll drawing method at the draw ratio in the longitudinal direction of 3.5 times so as to allow the thickness of the film after drawing to be 12 μm. After lengthwise drawing, the film was cooled by a cooling roll whose surface temperature was set to 25° C., and then wound as a roll. The resulting film was evaluated for various properties in the above-mentioned manner. The evaluation results are shown in Table 3. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

TABLE 3 Example Example Example Example Example Example 1 2 3 4 5 6 layer layer stracture single single single single two-type two-type stracture layer layer layer layer two-layer two-layer core layer A A A A C C skin layer A A ratio of — — — — 2/8 2/8 skin/core MD ratio 3.5 4.5 5.5 6 3.5 4.5 film thickness (μm) 12 12 12 12 12 12 Heat longitudinal 50 60 67 70 55 64 Shrinkage direction in hot water width direction 6 8 8.2 9 7 8.1 of 98° C. (%) Shrinkage maximum 9.5 10.2 11.2 12 8 8.5 Stress (MPa) under hot after 30 seconds 7 9.2 10.7 11.5 6.1 8.1 air of 90° C. (MPa) stress ratio 0.74 0.90 0.96 0.96 0.76 0.95 Refractive longitudinal 1.6176 1.6501 1.6698 1.6823 1.6055 1.6233 Index direction Shrinkage wrinkle Good Good Good Good Good Good Finising sink mark Good Good Good Good Good Good Property shrinkage Good Good Good Good Good Good shortage slack Good Good Good Good Good Good Example Example Example Example Example Example 7 8 9 10 11 12 layer layer stracture two-type two-type two-type two-type two-type two-type stracture two-layer two-layer two-layer two-layer two-layer two-layer core layer C C C C C C skin layer A A B B B B ratio of 2/8 2/8 2/8 2/8 2/8 2/8 skin/core MD ratio 5.5 6 3.5 4.5 5.5 6 film thickness (μm) 12 12 12 12 12 12 Heat longitudinal 69 71 49 58 62 68 Shrinkage direction in hot water width direction 8.5 9 5.8 7.5 7.9 8.2 of 98° C. (%) Shrinkage maximum 9 9.4 9.4 9.7 10.5 12.2 Stress (MPa) under hot after 30 seconds 8.8 9.2 8.2 9 10.1 11.9 air of 90° C. (MPa) stress ratio 0.98 0.98 0.87 0.93 0.96 0.98 Refractive longitudinal 1.6452 1.6700 1.6104 1.6305 1.6511 1.6724 Index direction Shrinkage wrinkle Good Good Good Good Good Good Finising sink mark Good Good Good Good Good Good Property shrinkage Good Good Good Good Good Good shortage slack Good Good Good Good Good Good

Example 2

A film was produced in the same manner as that in Example 1 except that the draw ratio in the longitudinal direction was set to 4.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 3

A film was produced in the same manner as that in Example 1 except that the draw ratio in the longitudinal direction was set to 5.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 4

A film was produced in the same manner as that in Example 1 except that the draw ratio in the longitudinal direction was set to 6 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 5

Polyester 1, polyester 2, and polyester 3 as described above were mixed in the mass ratio of 45:5:50 to prepare a mixed resin for a skin layer. Polyester 1, polyester 2, polyester 3 and polyester 4 as described above were mixed in the mass ratio of 25:5:60:10 to prepare a mixed resin for a core layer. The mixed resin for a skin layer and the mixed resin for a core layer were co-extruded at a temperature of 280° C. using a T-die mold equipped with a two-layer multi-manifold using two biaxial extruders, and quenched with a cooling roll to produce a two-layered sheet of a skin layer/a core layer. At this time, the mixed resins were co-extruded such that the thickness ratio of the skin layer and the core layer was the skin layer: the core layer=2:8. Next, the obtained sheet was heated to 80° C., and then lengthwise drawn by a roll drawing method at the draw ratio in the longitudinal direction of 3.5 times so as to allow the entire thickness of the film after drawing to be 12 μm. The film after lengthwise drawing was cooled by a cooling roll, and then wound as a roll. The resulting film was evaluated for various properties in the above-mentioned manner. The evaluation results are shown in Table 3. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 6

A film was produced in the same manner as that in Example 5 except that the draw ratio in the longitudinal direction was set to 4.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 7

A film was produced in the same manner as that in Example 5 except that the draw ratio in the longitudinal direction was set to 5.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 8

A film was produced in the same manner as that in Example 5 except that the draw ratio in the longitudinal direction was set to 6 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 9

A film was produced in the same manner as that in Example 5 except that polyester 1, polyester 2, and polyester 3 as described above were mixed in the mass ratio of 70:5:25 to prepare a mixed resin for a skin layer. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 10

A film was produced in the same manner as that in Example 9 except that the draw ratio in the longitudinal direction was set to 4.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 11

A film was produced in the same manner as that in Example 9 except that the draw ratio in the longitudinal direction was set to 5.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 12

A film was produced in the same manner as that in Example 9 except that the draw ratio in the longitudinal direction was set to 6 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 13

A film was produced in the same manner as that in Example 9 except that polyester 1, polyester 2, polyester 3, and polyester 4 as described above were mixed in the mass ratio of 5:5:66:24 to prepare a mixed resin for a core layer. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property. The evaluation results are show in Table 4.

Example 14

A film was produced in the same manner as that in Example 13 except that the draw ratio in the longitudinal direction was set to 4.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 15

A film was produced in the same manner as that in Example 13 except that the draw ratio in the longitudinal direction was set to 5.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 16

A film was produced in the same manner as that in Example 13 except that the draw ratio in the longitudinal direction was set to 6 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 17

Polyester 1, polyester 2, and polyester 3 as described above were mixed in the mass ratio of 45:5:50 to prepare a mixed resin for a skin layer. Polyester 1, polyester 2, polyester 3 and polyester 4 as described above were mixed in the mass ratio of 25:5:60:10 to prepare a mixed resin for a core layer. The mixed resin for a skin layer and the mixed resin for a core layer were co-extruded at a temperature of 280° C. using a T-die mold equipped with a three-layer multi-manifold using two biaxial extruders, and quenched with a cooling roll to produce a three-layered sheet of a skin layer/a core layer/a skin layer. At this time, the mixed resins were co-extruded such that the thickness ratio of the skin layer and the core layer was the skin layer:the core layer:the skin layer=1:8:1. Next, the obtained sheet was heated to 80° C., and then lengthwise drawn by a roll drawing method at the draw ratio in the longitudinal direction of 4.5 times so as to allow the entire thickness of the film after drawing to be 12 μm. The film after lengthwise drawing was cooled by a cooling roll, and then wound as a roll. The resulting film was evaluated for various properties in the above-mentioned manner. The evaluation results are shown in Table 4. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 18

A film was produced in the same manner as that in Example 17 except that polyester 1, polyester 2, and polyester 3 as described above were mixed in the mass ratio of 70:5:25 to prepare a mixed resin for a skin layer. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Example 19

A film was produced in the same manner as that in Example 18 except that polyester 1, polyester 2, polyester 3, and polyester 4 as described above were mixed in the mass ratio of 5:5:66:24 to prepare a mixed resin for a core layer. As a result of the evaluation, the film had adequate shrinkage property and good shrinkage finishing property.

Comparative Example 1

A film was produced in the same manner as that in Example 1 except that the draw ratio in the longitudinal direction was set to 2 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the banding film after shrinkage slackened, and thus the film had poor shrinkage finishing property.

Comparative Example 2

A film was produced in the same manner as that in Example 1 except that polyester 1, polyester 2 and polyester 3 as described above were mixed in the mass ratio of 70:5:25 and the mixed resin was introduced into an extruder, the draw ratio in the longitudinal direction was set to 2.5 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the banding film after shrinkage did not have enough amount of shrinkage, and thus the film had poor shrinkage finishing property.

Comparative Example 3

A film was produced in the same manner as that in Example 2 except that polyester 1, polyester 2, polyester 3 and polyester 4 as described above were mixed in the mass ratio of 25:5:60:10 and the mixed resin was introduced into an extruder. As a result of the evaluation, the label after shrinkage slackened, and thus the film had poor shrinkage finishing property.

Comparative Example 4

A film was produced in the same manner as that in Example 2 except that polyester 1, polyester 2, polyester 3 and polyester 4 as described above were mixed in the mass ratio of 5:5:66:24 and the mixed resin was introduced into an extruder. As a result of the evaluation, the banding film after shrinkage slackened, and thus the film had poor shrinkage finishing property.

Comparative Example 5

A film was produced in the same manner as that in Comparative Example 4 except that the draw ratio in the longitudinal direction was set to 3 times, and the amount of the molten mixed resin extruded from a T-die was adjusted so as to allow the thickness of the film after drawing in the longitudinal direction to be 12 μm. As a result of the evaluation, the banding film after shrinkage slackened, and thus the film had poor shrinkage finishing property

TABLE 4 Comparative Example Example Example Example Example Example Example 13 14 15 16 17 18 19 layer layer stracture two-type two-type two-type two-type two-type two-type two-type stracture two-layer two-layer two-layer two-layer three-layer three-layer three-layer core layer D D D D C C D skin layer B B B B A B B ratio of 2/8 2/8 2/8 2/8 1/8/1 1/8/1 1/8/1 skin/core MD ratio 3.5 4.5 5.5 6 4.5 4.5 4.5 film thickness (μm) 12 12 12 12 12 12 12 Heat longitudinal 54 65 70 71 64 58 65 Shrinkage direction in hot water width direction 7 8.2 8.5 8.5 8.1 7.5 8.2 of 98° C. (%) Shrinkage maximum 8.46 8.73 9.45 10.98 8.5 9.7 9.9 Stress (MPa) under hot after 30 seconds 7.38 8.1 9.09 10.71 8 7.6 7.9 air of 90° C. (MPa) stress ratio 0.87 0.93 0.96 0.98 0.94 0.78 0.80 Refractive longitudinal 1.6092 1.6301 1.6521 1.6712 1.6233 1.6305 1.6301 Index direction Shrinkage wrinkle Good Good Good Good Good Good Good Finising sink mark Good Good Good Good Good Good Good Property shrinkage Good Good Good Good Good Good Good shortage slack Good Good Good Good Good Good Good Comparative Comparative Comparative Comparative Comparative Example Example Example Example Example 1 2 3 4 5 layer layer stracture single single single single single stracture layer layer layer layer layer core layer A B C D D skin layer ratio of — — — — — skin/core MD ratio 2 2.5 4.5 4.5 3 film thickness (μm) 12 12 12 12 12 Heat longitudinal 43 35 70 75 55 Shrinkage direction in hot water width direction 4 3 5 10 3 of 98° C. (%) Shrinkage maximum 6 9 5.5 4.7 3.5 Stress (MPa) under hot after 30 seconds 3.1 7.5 3.2 1.8 1.2 air of 90° C. (MPa) stress ratio 0.52 0.83 0.58 0.38 0.34 Refractive longitudinal 1.5880 1.5860 1.6481 1.6462 1.5860 Index direction Shrinkage wrinkle Good — Good Good Good Finising sink mark Good — Good Good Good Property shrinkage Good Poor Good Good Good shortage slack Poor — Poor Poor Poor

INDUSTRIAL APPLICABILITY

The heat-shrinkable polyester film of the present invention has excellent properties as describe above and thus can be used suitably as a label application and a banding application to bind a box lunch container or the like. A package such as a bottle obtained by using the heat-shrinkable polyester film of the present invention as a label or a box lunch container obtained by using the heat-shrinkable polyester film of the present invention as a banding film shows a good appearance. 

1. A heat-shrinkable polyester film which has a main shrinkage direction in a longitudinal direction of the film and satisfies the following requirements (1) to (4): (1) the film has a hot-water shrinkage in the main shrinkage direction of the film of 40% or more and 80% or less when treated for 10 seconds in hot water of 98° C.; (2) the film has a hot-water shrinkage in a direction orthogonal to the main shrinkage direction of the film of −5% or more and 15% or less when treated for 10 seconds in hot water of 98° C.; (3) with regard to a shrinkage stress in the main shrinkage direction of the film measured under hot air of 90° C., the film has a shrinkage stress ratio of 0.6 or more and 1.0 or less, the shrinkage stress ratio being represented by the following equation: shrinkage stress ratio=(shrinkage stress after 30 seconds)÷(maximum shrinkage stress); and (4) the film has a refractive index in the main shrinkage direction of the film of 1.600 or more.
 2. The heat-shrinkable polyester film according to claim 1, wherein a maximum shrinkage stress in the main shrinkage direction of the film measured under hot air of 90° C. is 15 MPa or less.
 3. The heat-shrinkable polyester film according to claim 2, wherein the film comprises ethylene terephthalate as a main constituent component and 2 mol % or more of at least one monomer component that can form an amorphous component in a whole polyester resin component.
 4. The heat-shrinkable polyester film according to claim 3, wherein neopentyl glycol is used as at least one monomer component that can form an amorphous component.
 5. A package obtained by covering at least a part of an outer periphery of an object to be packaged with the heat-shrinkable polyester film according to claim 4 and then shrinking the film on the covered object by heat.
 6. The heat-shrinkable polyester film according to claim 2, wherein neopentyl glycol is used as at least one monomer component that can form an amorphous component.
 7. The heat-shrinkable polyester film according to claim 1, wherein the film comprises ethylene terephthalate as a main constituent component and 2 mol % or more of at least one monomer component that can form an amorphous component in a whole polyester resin component.
 8. The heat-shrinkable polyester film according to claim 7, wherein neopentyl glycol is used as at least one monomer component that can form an amorphous component.
 9. The heat-shrinkable polyester film according to claim 1, wherein neopentyl glycol is used as at least one monomer component that can form an amorphous component.
 10. A package obtained by covering at least a part of an outer periphery of an object to be packaged with the heat-shrinkable polyester film according to claim 1 and then shrinking the film on the covered object by heat. 